Advice for providing power to a barn conversion

[The General]

Evening all - I'm hoping to run something by a few brains to make sure I've not forgetten anything or done anything stupid! Apologies if it's a bit of an essay but I'll try and give you as much information as possible. Thanks for your time.

Right, here goes.

Existing set-up was a house with a small barn which has some lights and a couple of sockets. Power was coming from the main house (TT) being fed by an RCD protected consumer unit on a 30A MCB into 4mm cable out to a waterproof box outside. From this box it was then connected to some 16mm 2-core armoured for an underground run across to the barn.

So, clients have decided to convert barn to a small holiday letting and (as there's no gas/oil) want to have an electric shower, two instantaneous water heaters, oven, d/washer, w/machine...etc. They want to re-use the armoured cable (to avoid digging more trenches) but obviously I've said that the existing feed isn't going to do the trick - and this is where I need your input to make sure my solution makes sense.

Proposed plan:
RDL blocks in the main cut-out box (already there) and come out in 25mm tails into the house - straight into an 80A switch fuse. (There is about 5m of cable run through the house to get to the outside wall) From the switch fuse I intend to run 16mm tails through the house (behind kitchen cupboards, not buried) until it gets outside to where the existing 16mm armoured is terminated. Connect to the armoured across to the barn and then terminate into a new consumer unit. Barn will have it's own earth rod, and I was planning to connect the armoured sheathing to earth at this end as there isn't a nearby earth to use at the main house where the armoured terminates. Hope that makes sense?!?

New C/U will probably be M/S with RCBOs if that's of interest.

Hope that's enough information for everyone - let me know if I've missed anything obvious!

Thanks guys/gals.

 

[davesparks]

You've missed something very obvious; you can't earth the armouring at the barn end only! It needs to be connected to the earthing system of the supply end.
I don't understand your comment about there not being a nearby earth to use as you will be running a CPC from the new switch-fuse to the proposed joint anyway won't you?
Also you may need a 100mA RCD on the sub main unless you have a very good Ra at the house.

Personally I would extend the existing SWA with another bit of SWA and a cast joint and take that directly to the switch-fuse.

Also I would want to test that existing SWA fully before comitting to using it again.

This is all assuming you have done the calculations for the sub-main to check that 16mm is the right size for the job?

 

[sparks1234]

Yes, you need to protect the cable so bond armour at supply end, how far is the run

 

[The General]

You've missed something very obvious; you can't earth the armouring at the barn end only! It needs to be connected to the earthing system of the supply end.
I don't understand your comment about there not being a nearby earth to use as you will be running a CPC from the new switch-fuse to the proposed joint anyway won't you?
Not in my head - I was picturing L&N tails out of the meter into a switch fuse, there isn't an earth involved until you get to the MET where the Rod is connected? Am I being daft?

Also you may need a 100mA RCD on the sub main unless you have a very good Ra at the house.
Not checked yet - will do!

Personally I would extend the existing SWA with another bit of SWA and a cast joint and take that directly to the switch-fuse.

Also I would want to test that existing SWA fully before comitting to using it again.
I/R > 1000Mohm L/N L/E and N/E - phew!

This is all assuming you have done the calculations for the sub-main to check that 16mm is the right size for the job?
If you're talking about total loading, then yes, it's close, but okay

I'm still relatively new to the whole 'design' part of the job - historically I've been taking orders from someone else who's decided what's going where & how, hence why I'm trying to ask questions and make sure I'm not getting it wrong....apparently I may have a significant gap in my knowledge....rustling pages of BGB this weekend I feel....

 

[The General]

Yes, you need to protect the cable so bond armour at supply end, how far is the run

Total run from cut-out to new C/Unit (including armoured) is probably no more than 8m.

Just realised - could I use the earth on the old 4mm feed to earth the armoured at supply side?

 

[davesparks]

I think you've missed a fundamental point here, a basic circuit such as this comprises three conductors, live, neutral and CPC. Wherever the live and neutral go the CPC must follow! You don't just run L and N without a CPC and pick up any old earth you can find near to where they are connected.

That is to say you need to run an appropriately sized CPC from the switch fuse (or the MET if the switch fuse is at the origin) and it must go everywhere the L and N go.

You need to calculate the size of the either by use of the adiabatic equation or via selection according to the table in the regs (answer is 16mm cpc from the table)

Have you only tested IR of the SWA? You need to check impedance aswell, and personally i'd want a high current earth loop impedance test to ensure the armour hasn't corroded anywhere along its length.

I wasn't talking about total loading for the calculations, I meant have you checked the cable size based on In taking in to account all relevant de-rating factors, and calculated the volt drop?

 

[davesparks]

could I use the earth on the old 4mm feed to earth the armoured at supply side?

No, you are using the armoured as the CPC not just bonding it!

 

[The General]

I think you've missed a fundamental point here, a basic circuit such as this comprises three conductors, live, neutral and CPC. Wherever the live and neutral go the CPC must follow! You don't just run L and N without a CPC and pick up any old earth you can find near to where they are connected.

That is to say you need to run an appropriately sized CPC from the switch fuse (or the MET if the switch fuse is at the origin) and it must go everywhere the L and N go.

Now I'm confused. In a typical TT installation: From the DNO meter box there are L&N cables going to the main switch in C/U. Earth is provided by Rod connected to MET in C/U - there isn't a CPC provided to 'go with' the L&N between the meter and the C/U. Surely what I'm proposing is just a longer version of this? L&N tails going from the meter to a different C/U further away?

I may have had too many glasses of wine already..it is Friday...but I'm beginning to question if this is the right career for me, sounds like I've totally got the wrong end of the stick somewhere along the line. :sad3:

 

[davesparks]

No, what you are proposing is a distribution circuit which starts at the switch-fuse and runs to the new CU in the barn.

The earth from the rod is the earth that 'goes with' the L and N tails, but it is not a CPC it is the earthing conductor.

 

[The General]

No, what you are proposing is a distribution circuit which starts at the switch-fuse and runs to the new CU in the barn.

The earth from the rod is the earth that 'goes with' the L and N tails, but it is not a CPC it is the earthing conductor.

Okay, so I could take a CPC by connecting to the rod at the main house (which is pretty close to where the switchfuse would be) and run that with the tails across to where the armoured starts and connect it to the sheathing?

 

[davesparks]

No, the earthing conductor connects the rod to the MET, all cpc's and bonding conductors then connect back to the MET.

You install a switch fuse at the intake position as you have described, this has an L and N tail in to it and also an earth from the MET. You then connect an outgoing cable comprising L, N and CPC from the switchfuse to the CU in the barn.

If you are going to extend a cable you should use the same type and size of cable. ie. To extend SWA use SWA, to extend MICC use MICC, to extend steel conduit use steel conduit etc.

Also you don't connect to the sheathing, you connect to the armouring (the sheathing is the black plastic outer of the cable) via an appropriate SWA gland (not an earth clamp or any other dodgy method)

 

[The General]

No, the earthing conductor connects the rod to the MET, all cpc's and bonding conductors then connect back to the MET.

You install a switch fuse at the intake position as you have described, this has an L and N tail in to it and also an earth from the MET. You then connect an outgoing cable comprising L, N and CPC from the switchfuse to the CU in the barn.

If you are going to extend a cable you should use the same type and size of cable. ie. To extend SWA use SWA, to extend MICC use MICC, to extend steel conduit use steel conduit etc.

Also you don't connect to the sheathing, you connect to the armouring (the sheathing is the black plastic outer of the cable) via an appropriate SWA gland (not an earth clamp or any other dodgy method)

Doh! I knew what I meant

 

[Rockingit]

I'm going to throw a swerve ball in this; if I'm reading and understanding the OP correctly then he's talking about taking the feed from the meter, splitting it, one goes to house via tails and one goes to barn via a KMF. Divorced earth at the barn end and the armour for the SWA only glanded at that end as well.

Personally, can't see any big issues with that. BGB doesn't say WHERE an armour has to be earthed, just it does. The swa becomes a 5s circuit anyway as it's not a final.

 

[sparks1234]

I'm going to throw a swerve ball in this; if I'm reading and understanding the OP correctly then he's talking about taking the feed from the meter, splitting it, one goes to house via tails and one goes to barn via a KMF. Divorced earth at the barn end and the armour for the SWA only glanded at that end as well.

Personally, can't see any big issues with that. BGB doesn't say WHERE an armour has to be earthed, just it does. The swa becomes a 5s circuit anyway as it's not a final.

Just to clear my mind, are you creating a TT system within the barn?

 

[Rockingit]

Just to clear my mind, are you creating a TT system within the barn?

That was my reading of it, yes. One supply, 2 x TT's.

You'd need to have your standard 30mA RCD setup each sub.

 

[The General]

I'm going to throw a swerve ball in this; if I'm reading and understanding the OP correctly then he's talking about taking the feed from the meter, splitting it, one goes to house via tails and one goes to barn via a KMF. Divorced earth at the barn end and the armour for the SWA only glanded at that end as well.

Personally, can't see any big issues with that. BGB doesn't say WHERE an armour has to be earthed, just it does. The swa becomes a 5s circuit anyway as it's not a final.

That' exactly what I'm talking about!
I was lying awake in bed last night and I couldn't think of any good reason why the armour has to be earthed at the supply side. In reality, earthing (via MET) to a rod on the supply side, or eathing (via MET) to a rod at the barn - and the two rods are less than 4ft away from each other!

 

[The General]

That was my reading of it, yes. One supply, 2 x TT's.

You'd need to have your standard 30mA RCD setup each sub.

Yes, TT for the main house, and another rod for the barn.
I was planning to use 30mA RCBOs for each circuit.

 

[Rockingit]

That' exactly the two rods are less than 4ft away from each other!

That's the only thing I'd raise an eyebrow to, as they will interfere with each other. See if you can get them further apart.

 

[The General]

That's the only thing I'd raise an eyebrow to, as they will interfere with each other. See if you can get them further apart.

Hmm. Not considered that. Clients are quite specific about what they do an don't like so the location of the new rod was decided by them. If there'd been any choice in the matter I'd have run some new armour clipped to the underside of a raised step across the path straight into the meter box - but that apparently wasn't acceptable aesthetically.....

Anyhoo.
How far apart would they need to be?

 

[Rockingit]

How far apart would they need to be?

Ooh....that's a good question! Off the top of my head I'd say greater than 3xdepth of rod, but happy to be corrected! Nowhere near a BGB all day.